Lignocellulose is an inexpensive and abundant inedible biomass source and consists of three main components: cellulose, hemicellulose, and lignin. Lignin is used as a fuel for direct burning. It is desired that lactic acid is produced from the two other components, cellulose and hemicellulose. Cellulose consists of glucose, which is available for lactic acid bacteria, while hemicellulose consists primarily of pentoses, such as xylose and arabinose, which are unavailable for most lactic acid bacteria. Accordingly, development of a technique for lactic acid production from a pentose is demanded.
To date, some lactic acid bacteria, such as Lactobacillus pentosus (Non-Patent Document 1), Lactobacillus brevis (Non-Patent Document 2), Lactobacillus plantarum (Non-Patent Document 3), and Leuconostoc lactis (Non-Patent Document 4), have been reported to utilize either or both of xylose and arabinose. Such lactic acid bacteria produce equimolar amounts of lactic acid and acetic acid simultaneously, which is called “heterolactic fermentation”, without known exception.
FIG. 1 shows a scheme of pathways for xylose and arabinose utilization. As shown in FIG. 1, xylose and arabinose are converted to xylulose 5-phosphate (X5P) by a series of intracellular metabolic enzymes in a lactic acid bacterium. During heterolactic fermentation, X5P is then cleaved by an action of a phosphoketolase into equimolar amounts of glyceraldehyde 3-phosphate (GAP) and acetyl phosphate (Acetyl-P), which are eventually converted to lactate and acetate, respectively (Non-Patent Document 5). This pathway is called a phosphoketolase (PK) pathway. Meanwhile, also known as a pathway for lactic fermentation by pentose utilization is a pentose phosphate (PP) pathway, in which GAP and fructose 6-phosphosphate (F6P) are yielded from X5P, and lactate alone is eventually produced. This is called “homolactic fermentation.”
Lactococcus lactis IO-1 is known to have the PP pathway in addition to the PK pathway and produce lactic acid at more than the equimolar amount, however, complete homolactic fermentation has not been achieved (Non-Patent Document 5). No lactic acid bacterium that is capable of homolactic fermentation from a pentose has been reported so far.
It has been reported that Lactobacillus pentosus loses its ability to grow on pentoses due to a defect in a gene for phosphoketolase, an enzyme that acts in the PK pathway (Non-Patent Document 6). However, no report has described introduction of the PP pathway.